It is well known in the art how coal is created over geological time, including the byproducts of the coalification process: methane, carbon dioxide, hydrogen and other gases. The volume of methane thus generated is relatively large--a ton of anthracite today occupying a volume of less that 30 cubic feet is postulated to have generated in the order of 10,000 standard cubic feet of methane during its lifetime. Some of the early methane production, no doubt, bubbled up through the waters of ancient swamps and escaped to the air above. It is well known today, however, that many underground coal seams contain a volume of trapped methane (as expressed in standard cubic feet) many times the volume of the host coal. Cores taken from underground coal, when subjected to controlled desorption tests, often yield measured methane contents that correspond to more than 600 scf per ton of coal. Thus the coal seam is both a manufacturer of methane and a reservoir for methane storage. Methane in the coal seam reservoir is same as methane found in the petroleum industry in sandstone and carbonate reservoirs.
Petroleum reservoir engineering for natural gas (composed principally of methane) production is a well established art. Coal seam reservoir engineering for methane production is an emerging art and is significantly different from the relatively straightforward engineering problems of natural gas production. Both aarts deal with production of gases trapped in underground reservoirs. When there is a substantial amount of water also present in the underground reservoir, behavior of water during production of gas must be taken into account.
In petroleum reservoir engineering a water drive downdip in a natural gas reservoir generally serves to enhance production of natural gas. A production well drilled into an updip location within the underground reservoir provides a lower pressure outlet for trapped natural gas, which flows readily to the wellhead following Darcy's Law. A routine drillstem test confirms such flow prior to the production phase.
Coal seam reservoir engineering faces more complex problems when the coal bed is an aquifer. Compared to a sandstone natural gas reservoir of the same depth, the coal seam methane reservoir tends to be relatively underpressured, and the water is located throughout the coal seam rather than being conveniently located out of the way downdip. In the sandstone natural gas reservoir porosities and permeabilities are relatively good, while the coal bed porosities and the permeabilities are relatively poor by comparison. In fact most of the methane in coal is trapped by adsorption on the enormous square footage of internal surfaces within the micropore system of the coal itself. A routing drillstream test of the coal seam, at best, will show only a small quantity of methane that flows from the fractures in the coal--but, in most cases, will show no methane at all. Thus water throughout the coal under hydraulic head pressure inhibits the two phase methane flow.
A partial reduction of hydraulic head within a gassy coal seam may permit the flow of methane from the natural fracture system, but this flow is a relatively small portion of the methane in place. This type of flow follows Darcy's Law. Adsorbed methane in the micropores, the bulk of the methane present, must be desorbed for initiation of flow, following Fick's Law of diffusion. This requires removal of all or substantially all of the hydraulic head from the vicinity of the wellbore. The two-phase steps of methane flow are desorption and flow to the fracture system (Fick's Law) and flow through the fracture system to the wellbore (Darcy's Law).
Production rates often can be increased substantially for natural gas by hydraulic fracturing of the reservoir. If the reservoir is a sandstone with low permeability, good results can be obtained by adding relatively coarse grained sand to the fracturing fluid, the sand particles serving as props to keep the fractures open. Likewise, production rates for methane drainage from coal can be increased by fracturing, but the fracturing procedures must be tailored to the special features of the coal bed. Lower rank coals are relatively soft and pliable compared to sandstone. A massive sand frac into coal may cause more problems than it solves. For example, the coal around the natural fracture system may be pulverized to the point where large amounts of coal fines accompany fluids flow to the wellbore, and a substantial amount of the frac sand may also return to the wellbore in the same manner. In the case of a high voltage content coal, fracturing pressures may cause the volatile portion of the coal to ooze into the natural fracture system, thus decreasing instead of increasing permeability as planned.
The water in a wet seam arrived in its present position by migrating through the existing fracture system of the coal. Since this water must be substantially removed for effective methane production, a great deal of useful data can be gained from the water itself. If the water is potable, its source is probably from a distant outcrop of the coal--useful information when compared with information related to pumping rates needed to remove hydraulic head. This newly acquired data may indicate that the existing fracture system required little or no further stimulation. If the water contains a considerable amount of dissolved solids, its source probably is from remnants of an ancient ocean, which if nearby certainly should not be further connected by additional fracturing.
Looking again to the differences between natural gas reservoir production and coal bed methane production, a natural gas well typically is drilled through the carbonate or sandstone rock reservoir. A drillstream test is made to confirm that gas is present. Then well logs are run and casing is set to a location at or below the bottom of the reservoir. From well logs optimum locations for perforations are selected and the casing is perforated. With a water drive downdip, the well will clean up in a relatively short time with maximum production rate attained, followed by gradual reduction of production rates over an extended period of time measured in years. A similar well for coal bed methane production would be drilled through the coal seam and into the underlying stratum. Preferably the well would be cored from a point above the coal seam, through the seam and to a point below the seam. Cores of the coal would be subjected to controlled desorption tests to ascertain methane content. Casing would be set, preferably to the top of the seam for a "barefoot" completion with open hole through the coal. A pump then would be set, preferably below the coal to avoid ascending gas bubbles that would vaporlock the pump. Upon opening the well, water would rise in the wellbore until static head level is attained--a point that could be 100 feet or so below the wellhead. Well logs could be run at an appropriate time during the drilling sequence for accurate determination of coal seam location, but there are no well logs available currently that can detect the presence or absence of methane in the coal. With the well open at the wellhead and the column of water at the static head level, typically no methane is produced, so there is no well cleanup at this point and no indication of what the production rate curve may be. To attain well cleanup, methane production and an indication of the true form of the production rate curve, hydraulic head must be reduced in the vicinity of the wellbore.
At the present state of the art for coal bed methane reservoir engineering, hydraulic head is removed by the simple expedient of extensive pumping operations. Water lifting operations may involve production of 200 or more barrels per day for a year or more before well cleanup begins. During well cleanup typically the first methane is produced as a flow from the fracture system. This methane flow generally is of short duration, a matter of days, fitfully initially, followed by a relatively strong blow, then a relatively sharp decline. Water pumping must continue to maintain water drawdown that permits continuing cleanup of the micropore system adjacent to the coal fracture system. This initiates desorption of methane from the micropores and begins the sustained production to be expected from the well. Complete well cleanup requires an extended period of time, compared to the relatively short period required for a natural gas well. Consequently, the production rate curves are quite different for these two types of wells.
The production rate curve for the natural gas well, after faltering somewhat during a brief well cleanup period, rapidly reaches a peak production rate that may remain relatively flat for a period of time, followed by a gradual decline over a long period of time. Typically there is no water production until near the end of commercial production.
The production rate curve for the wet coal bed methane well shows a brief burst of production (free methane in the fracture system) followed by a lull in production, followed by sustained production at a low rate, with ever increasing production rates to a peak rate many years later. Initially, water production rates are relatively high and then decline as methane production rates increase. It is postulated that once a coal bed methane well reaches peak production rate, a decline will set in, comparable to that of a natural gas well; however, no wells so far have been in production long enough to verify this projection. Likewise, it is postulated that coal bed water production will decline to zero, or near zero, at some point in time, long before the methane well reaches economic depletion.
A new discovery of natural gas can be confirmed immediately upon completion of a drillstem test. Determination of the true economic significance, however, must await production performance over a period of time to determine the projected volume of the reservoir and the projected rates of recovery. A new discovery of wet coal bed methane can be confirmed upon completion of desorption tests on cores. Determination of its economic significance, likewise, must await future events. The coal bed reservoir engineer would, as a minimum, like to see production rate curves for the initial temporary production, the beginning of sustained production, and more particularly the slope of the sustained production rate curve--sometimes called the "reverse decline" curve. From an economic point of view, it would be advantageous to see these segments of the curves before a lengthy and costly water pumping operation is undertaken.
It is an object of the present invention to teach methods of dewatering a coal seam within the vicinity of the wellbore, without resorting to conventional pumping, in order to establish early production and the resulting data therefrom. Other objects and advantages of the invention will become apparent as the description proceeds.